Objectives of this research are to examine the effects of aging gametes on the processes of fertilization and early embryo development. This includes slight hormonal imbalances at the time of fertilization which do not interfere with syngamy but affect subsequent development. Excellent embryo culture techniques permit parallel studies under highly controlled conditions to be carried out in vitro. Investigations specifically planned with rabbits and/or cattle are the following: 1. Post-ejaculatory deterioration of spermatozoa, especially the biochemical changes which occur during preservation by cryogenic means and are associated with fertilization failure and embryo development, will be studied. Certain sub-populations will be isolated for study. Possible means of prevention of damage will be examined simultaneously. 2. Hormonal imbalance at the time of fertilization or fertilization following withdrawal of steroid hormones after long-time therapy as these affect embryo development will be examined.